Size Distribution of Antarctic Micrometeorites

145 SIZE DISTRIBUTION OF ANTARCTIC MICROMETEORITES

S. Taylor1, G. Matrajt2 J.H. Lever1 D.J. Joswiak2 and D. E. Brownlee2 1Cold Regions Research and Engineering Laboratory, 72 Lyme Rd., Hanover NH 03755, USA [email protected] 2 Department of Astronomy, University of Washington, Seattle WA 98195 USA.

ABSTRACT heat exchanger precluded quantifying the unmelted Micrometeorites collected from the South Pole micrometeorite component. water well in 1995 and 2000 have terrestrial deposi- We have repeated the size distribution analyses tional ages of 1100-1500 AD and 800-1100 AD re- from samples collected from the SPWW plateau in spectively. We measured a size distribution for micro- 2000. Between 1995 and 2000 the bottom of the well meteorites collected in 2000. Compared to the size deepened from 105 m to 134 m, which corresponds to distribution measured in 1995 the tail slopes of the size an ice depositional age of 800–1100 AD [3]. Because distributions are the same. However the 2000 collec- the entire plateau was vacuumed in 1995, the contami- tion has twice as many micrometeorites and propor- nation of the 2000 samples decreased by an order of tionally more large micrometeorites. magnitude and 1 to 5% of the samples were microme- We also tabulate the unmelted-to-melted ratio for teorites. We have identified and mounted 3272 melted micrometeorites in five size ranges and compare the and unmelted micrometeorites from the plateau sam- results with previous collections. The unmelted-to- ple. In this paper we compare the size distribution for melted ratios obtained for this collection are similar to the 2000 plateau sample with the one obtained in 1995. those from Greenland and an Antarctic collection of We also tabulate the unmelted-to-melted ratio for mi- present-day micrometeorites. Collections from older crometeorites in five size ranges and compare the re- Antarctic ice show larger and more variable ratios. sults with previous collections.

1. INTRODUCTION 2. SAMPLES and METHODS Micrometeorites are terrestrially collected extra- The samples were collected in Nov 2000 from the terrestrial dust particles smaller than about two milli- SPWW, a 4,000-m3 reservoir melting pre-industrial meters. Their accretion rate, size distribution and com- ice. The well’s central plateau was vacuumed and position bear on numerous studies including deducing yielded ~ 10 g of material. We sieved this sample into the compositions of parent bodies, calibrating terres- >425, 250–425, 150–250, 106–150 and 53–106!m size trial sedimentation rates and interpreting the isotopic fractions. Using a binocular microscope we sorted record of seawater, linking influx to global climate 100% of the >150-!m fractions, 29% of the 106-150 change, and assessing the role of extraterrestrial mate- fraction and 8.9% of the 53–106!m fraction and rials in life processes. removed all potential ET grains. Altogether we In 1995, Taylor et al. [1] retrieved ~ 200g of mate- mounted in epoxy and sectioned over 4000 particles. rial from the bottom of the South Pole water well Using a SEM/EDAX we checked each particle for (SPWW) of which about 0.1% were cosmic spherules composition and found that 3272 of particles had (melted micrometeorites). They vacuumed an elevated chondritic compositions and textures that were central ice plateau and sculpted pockets surrounding consistent with those seen in micrometeorites. the plateau (which were enriched in terrestrial and ex- Transmitted light microscopy was used to measure the traterrestrial materials). Taylor et al. [1] argued that major and minor axis of each micrometeorite and both the plateau contained only micrometeorites deposited transmitted and reflected light was used to classify in the snow and ice directly above it and that the pla- them based on their cross-sectional textures. teau neither lost nor gained micrometeorites from other parts of the well. They measured the particle size dis- 3. RESULTS and DISCUSSION tribution of cosmic spherules in the plateau sample, the 3.1 Size Distributions area of the plateau, the age of ice melted (1100–1500 The cumulative size distributions for the 1995 and AD) and computed a terrestrial accretion rate for cos- for the 2000 plateau samples are shown in Figure 1. mic spherules 50–700 !m in diameter of 1600 ± 300 The numbers of micrometeorites in the smallest two tons/yr [1] or 4 ± 2 percent of the flux measured above size fractions of the 2000 sample have been increased the atmosphere [2]. Iron oxide grains from the well’s to account for the fact that only a portion of each size fraction was sorted. The best-fit line to the tail of the

Proc. ‘Dust in Planetary Systems’, Kauai, Hawaii, USA. 26--30 September 2005 (ESA SP-643, January 2007)

146 2000 plateau sample (d > 200 µm) has a slope of -5.4, 10000 slightly steeper than the -5.2 tail-slope calculated for cosmic spherules from the 1995 plateau sample [1]. Despite this similarity, the 2000 collection has propor- 1000 tionately more large (d > 200 µm) particles even Pocket, SP-95 though it includes unmelted micrometeorites. Interest- N (>d) ~ d-5.1 ingly a size distribution for 218 present-day cosmic 100 spherules collected from snow has a similar tail slope Pocket, SP-00 of -5.2 [4]. N (>d) ~ d-6.0 The 2000 plateau sample has 2.4 times more mi- 10 crometeorites than the 1995 collection, and because it Cumulative # of Micrometeorites has disproportionately larger particles, the difference 1 between the mass distributions and total extraterrestrial 100 1000 mass collected will be even larger. Unmelted microme- Average Diameter (µm) teorites in the 2000 sample, primarily in the smallest Figure 2. Cumulative size distributions for micrometeorites size fractions, account for about one-third of the num- from 1995 and 2000 pocket samples collected from the bot- ber increase but will have less effect on the total mass tom of the South Pole water well.

collected. The remainder could be due to the larger 3.2 Unmelted-to-melted ratio area vacuumed in 2000. We have classified all micrometeorites examined The SPWW increases slightly in diameter as it from the 2000 collection as either melted or unmelted. deepens. Preliminary analysis of scaled video records We count as unmelted the fine-grained and crystalline indicates that area vacuumed on the plateau in 2000 or coarse-grained unmelted micrometeorites (Figure was 45 ± 5 m2, compared with 17 ± 1 m2 in 1995. To- 3), the transitional scoriaceous and predominantly relic gether with the unmelted particles, this accounts for the grain bearing micrometeorites (Figure 4), and micro- number increase and might account for the larger mass meteorites that are composed of single mineral grains expected for the 2000 sample if both time periods ex- or possible chondrule fragments- note barred olivine perienced the same extraterrestrial influx. texture on upper left corner, scale bar =20!m (Figure It is possible that as the plateau area increased it 5). Melted micrometeorites are those rounded cosmic incorporated some of the surrounding pockets. These spherules that have relic grains in less than 50% of pockets have about an order-of-magnitude more mi- their cross sectional area (Figure 6). crometeorites per unit area and have similarly sized

particles as the plateau samples (Figure 2). However, we did not see remnant dark patches of concentrated particles on the plateau as would be expected if it incorporated adjacent pockets of material. Instead, it is likely that the outward circulation pattern in the SPWW [1] moves the pocket material outward as the plateau slowly expands.

10000 es t i r

o e t 1000 Figure 3. Unmelted micrometeorites fine-grained and coarse- e Plateau, SP-00 m -5.4 grained. o

r N (>d) ~ d c

i M

f 100 o

# Plateau, SP-95

-5.2 ve

i N (>d) ~ d t a l

u 10 m u C

1 100 1000 Average Diameter (µm) Figure 1. Cumulative size distributions for micrometeorites Figure 4. Transitional forms scoriaceous and relic grain from the 1995 and 2000 plateau samples collected from the bearing micrometeorites. bottom of the South Pole water well.

Proc. ‘Dust in Planetary Systems’, Kauai, Hawaii, USA. 26--30 September 2005 (ESA SP-643, January 2007)

147 has an unmelted-to-melted ratio of 0.44 across the range 53-250 !m, similar to the Greenland samples [5] for 50-300 µm. This value is also at the low end of the range found by Terada et al. [7] for 40-238 µm micrometeorites from 16-60 kyr ice. Similarly, the SPWW plateau has an unmelted-to- melted ratio of 0.34 across the range 53-425!m. This is an order-of-magnitude lower than results by Genge Figure 5. Single mineral and possible chondrule fragment. and Grady [8] and Walter et al. [9] for Cap Prud- homme samples of unknown age, although within a factor of 3 of the value obtained by Duprat et al [4] for 20-400!m present-day micrometeorites. Possible ex- planations include destruction or masking of unmelted micrometeorites in the SPWW and the Greenland samples, preferential concentration of unmelted micrometeorites in blue ice, variations in the types of micrometeorites being deposited over thousand year time scales, differences in the way each team classifies mi- Figure 6. Melted micrometeorites barred olivine and porphy- ritic spherules. crometeorites, and errors from small sample sizes. Each possibility is worth a brief discussion.

The numbers of melted and unmelted micromete- Table 1. Number of melted and unmelted micrometeorites in each size fraction are given in Table 1. As orites in different size ranges. expected, the proportion of unmelted micrometeorites Age Size generally increases as the size fraction decreases. We yr range # Un- # found very few unmelted micrometeorites in the Ref. x103 (!m) melted Melted U/M >250!m size fractions. Although we found a lower This study (SPWW 2000) unmelted-to-melted ratio in the 53-106 !m fraction 1.3 53-106 70 135 0.52 than the 106-150 µm fraction, this could reflect the 106-150 169 174 0.97 increased difficulty of identifying unmelted particles 150-250 134 1288 0.10 within the abundant terrestrial contamination in the 250-425 23 1138 0.02 smallest size fraction. >425 1 135 .0074 Table 1 also shows ratios of unmelted-to-melted Maurette et al. [5] micrometeorites reported by others. Maurette et al. [5] ~3 50-100 780 1780 0.44 found about half as many unmelted as melted micro-

meteorites in 50-300 !m samples collected from 100-200 265 570 0.46 Greenland ice. While Maurette et al. [6] found a simi- 200-300 45 101 0.45 lar ratio for 100-400 !m micrometeorites collected Maurette et al. [6] from blue ice at Cap Prudhomme Antarctica, they re- ? 50-100 >5 ported 5 times as many unmelted as melted microme- 100-400 ~0.3 teorites in the 50-100 !m size fraction from the same Terada et al. [7] collection. Terada et al. [7] sampled Antarctic blue ice 16 40-238 101 78 1.3 of three different ages and found a range in the un- 30 40-238 134 138 0.97 melted to melted ratio from 0.5 to 4.5 for micrometeor- 30 40-238 172 84 2.1 ites 40–238 !m in diameter. Genge and Grady [8] ex- 60 40-238 18 4 4.5 amined 550 micrometeorites in the size range 50-400 60 40-238 30 53 0.57 !m from Cap Prudhomme samples and found an un- Genge and Grady [8] melted to melted ratio of 3. Walter et al. [9] also stud- ? 50-400 412 138 3.0 ied micrometeorites collected Cap Prudhomme and Walter et al. [9] found an unmelted to melted ratio over 6. Duprat et al. ? 100-400 262 41 6.4 [4] collected 412 micrometeorites from new snow at Duprat et al. [4] Dome C and found about equal numbers of melted and 0 20-400 194 218 0.89 unmelted micrometeorites. To compare ratios across size fractions, we cor- Because of the excellent preservation of fragile rected the counts in the smallest fractions for the pro- particles in the SPWW collections, we do not think portions examined. The SPWW plateau sample thus that unmelted particles are destroyed during collection.

Proc. ‘Dust in Planetary Systems’, Kauai, Hawaii, USA. 26--30 September 2005 (ESA SP-643, January 2007)

148 Terrestrial contamination does slow the sorting process We are presently analyzing the distribution of mi- and increases the chance of overlooking unmelted micrometeorite types and converting sizes to masses to crometeorites, which are often dark and irregular in calculate a mass distribution and flux for the 2000 col- shape. To minimize this possibility two of us (ST and lection. When complete, these collection-level analy- GM) independently sorted each sample. ses will allow us and other investigators to place indi- It seems unlikely that unmelted micrometeorites vidual micrometeorites in context of the collection as a should be preferentially preserved in glacial blue ice whole and to explore interrelationships between mi- that has undergone significant deformation compared crometeorite types. In addition to comparing fluxes for with the firn and ice at South Pole. However, Maurette the 1995 and 2000 collections, we will look for differ- et al. [10] reported a micrometeorite concentrating ences in abundance of micrometeorite types to assess processes in Antarctic blue ice samples. It is possible whether extraterrestrial influx changed between the that the dark, irregular unmelted particles could be two sample periods. heated sufficiently to remain below the ice surface as the surface ablates, thereby preferentially concentrat- Acknowledgements: We thank NSF (Dr. Julie Palais pro- ing unmelted particles compared with that portion of gram manager) for funding the collection of micrometeorites comic spherules that are light-colored and smooth. from the South Pole water well and NASA (Dr. David Lind- strom program manager) for funding the analysis of the 2000 This process might explain the predominance of cos- collection. We also thank Sarah Elliott for imaging several mic spherules in Antarctic wind blow materials [11]. hundred of these micrometeorites, Dr. Charles Daghlian and We are classifying by type the SPWW 2000 sam- Dartmouth College for use of the SEM and Dr. Cecile En- ples and will compare the results with similar data grand for reviewing the paper and for her helpful sugges- from the 1995 samples to assess the possibility of tem- tions. poral variations in the types of materials arriving on Earth. This will also help us address the possibility of References: differences in extraterrestrial influx across 100-yr time [1] Taylor et al. Accretion rate of cosmic spherules measured scales. at the South Pole. Nature 392, 899-903, 1998. [2] Love S.G. and Brownlee D.E., A Direct measurement of Up to 20% of SPWW micrometeorites are transi- the terrestrial mass accretion rate of Cosmic dust, Sci- tional between melted and unmelted (Figure 4). How ence 262, 550-553, 1993. these are tallied will affect the unmelted-to-melted [3] Kuivinen et al., South Pole ice core drilling, 1981-1982. ratio reported by various research teams. A standard Antarctic Journal of the United States XVII, 89-91, classification system or clear descriptions of how one 1982. is classifying the particles would help reduce this [4] Duprat et al., Friable micrometeorites from central Ant- source of uncertainty. arctic snow, LPSC XXXVI, 1678.pdf, 2005. Lastly, considerable variability occurs within sub- [5] Maurette et al., Characteristics and mass distribution of samples from the same collections (see Table 1: Te- extraterrestrial dust from the Greenland ice cap, Nature, 328, 699-702, 1987. rada et al. [7] for duplicate samples from nominally the [6] Maurette et al. A collection of diverse micrometeorites same ice, and differences by Maurette et al [6], Genge recovered from 100 tonnes of Antarctic blue ice, Nature and Grady [8] and Walter et al [9] for Cap Prudhomme 351, 44-47, 1991. samples). Sufficient numbers of micrometeorites need [7] Terada et al. General characterization of Antarctic mi- to be examined to minimize this type of error. crometeorites collected by the 39th Japanese Research Expedition: Consortium studied of JARE AMMs (III), 4. CONCLUSIONS Anatarct. Meteorite Res. 14, 89-107, 2001. Micrometeorites from the SPWW collected in [8] Genge M.J. and Grady M.M., The Distribution of Aster- 1995 and 2000 arrived on Earth between 1100-1500 oids: Evidence from Antarctic Micrometeorites, LPSC AD and 800-1100 AD. The tail slopes of their size XXXIII, 1010.pdf, 2002. distributions are essentially the same, but the 2000 [9] Walter et al. The abundance of ordinary chondrite debris among Antarctic micrometeorites, Meteoritics 30, ab- collection had many more particles and proportionally stract 592-593, 1995. more large particles. The inclusion of unmelted mi- [10] Maurette et al. Cosmic dust in 50 kg blocks of blue ice crometeorites and the increase in area vacuumed could from Cap-Prudhomme and Queen Alexandra Range, account for these differences. Antarctica, Meteoritics 26, abstract, 257, 1992. Unmelted micrometeorites smaller than 150 µm [11] Harvey R. P. and Maurette M. The origin and signifi- are nearly as abundant as melted ones in the 2000 col- cance of cosmic dust from the Walcott Névé, Antarc- lection. The unmelted-to-melted ratios obtained for this tica. Proc. Lunar Planet. Sci. Conf. 21st, 569–578.1991. collection are similar to those from Greenland and an Antarctic collection of present-day micrometeorites. However, collections from older Antarctic ice show larger and more variable ratios.

Proc. ‘Dust in Planetary Systems’, Kauai, Hawaii, USA. 26--30 September 2005 (ESA SP-643, January 2007)